This invention relates to a process for gas cleanup to remove one or more metallic contaminants present as vapor. More particularly, the invention relates to a gas cleanup process using mass transfer to control the saturation levels such that essentially no particulates are formed, and the vapor condenses on the gas passage surfaces. It addresses the need to cleanup an inert gas contaminated with cadmium which may escape from the electrochemical processing of Integral Fast Reactor (IFR) fuel in a hot cell. The IFR is a complete, self-contained, sodium-cooled, pool-type fast reactor fueled with a metallic alloy of uranium, plutonium and zirconium, and is equipped with a close-coupled fuel cycle. Tests with a model have shown that removal of cadmium from argon gas is in the order of 99.99%. The invention could also apply to the industrial cleanup of air or other gases contaminated with zinc, lead, or mercury. In addition, the invention has application in the cleanup of other gas systems contaminated with metal vapors which may be toxic or unhealthy.
Pyrochemical processes are being developed to process Integral Fast Reactor (IFR) fuel material. The IFR fuel is a metallic uranium-plutonium-zirconium alloy clad in a stainless steel alloy. Electrorefining is the key step in the fuel processing cycle. In this operation, spent clad fuel is chopped and placed in baskets that are introduced into an electrorefiner (ER) vessel containing a molten LiCl-KCl salt mixture at 500.degree. C. A pool of cadmium lies beneath the molten salt. The basket is connected to a dc power supply and made anodic; nearly pure uranium is removed from the spent fuel by electrotransport to solid cathodes, then the plutonium and any remaining uranium in the feedstock are electrotransported to liquid cadmium cathodes. More than 99.9% of the actinides are removed from the cladding hulls, and noble metal fission products either remain in the basket or fall as particulate to the bottom of the electrorefiner. The alkali and alkaline earth metals in the spent fuel are oxidized and remain in the electrorefiner salt, as do most of the rare-earth fission products. Engineering-scale investigations of this reprocessing scheme are being conducted in a large, positive-pressure glove box containing a high-purity argon atmosphere. These studies are being carried out using depleted uranium and nonradioactive fission product materials, not plutonium or radioactive fission products.
During these investigations, some cadmium has been released from the ER into the glove box. The resulting aerosol plateout caused blackening of the walls of the glove box. In a companion study, the aerosols suspended in the glove box were characterized from gas samples withdrawn from a port near the top of the glove box on the wall adjacent to the ER. The gas samples were withdrawn at a rate of 2.8 L/min, passed through a 47-mm-dia Nucleopore filter (0.2-.mu.m pore size), and analyzed by X-ray diffraction (XRD) and by scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). The aerosols were found to consist mainly of cadmium, with some chlorine, but no potassium. No attempt was made to detect lithium in the aerosols. Aerosol concentration was highest immediately after opening the ER port. The primary particles were formed by homogeneous nucleation of vapor released from the ER and had a reference size of about 0.08 .mu.m. Formation of particles larger than 0.20 .mu.m was attributed to coagulation into agglomerates of the primary particles in the plume rising from the open ER port. Formation of particles exceeding 1 .mu.m suggested coagulation of the agglomerates. There was clear evidence that upon continued exposure, the agglomerates were sintered in the hot plume and were transformed into hexagonal crystals typical of cadmium. The sizes of the crystals reflected the size distribution of the agglomerates from which they were formed.
In a related study, cadmium vaporization was investigated on a laboratory scale using a 5.08-cm-dia crucible containing a molten cadmium pool (2.54-cm-high) covered with a molten salt (9.0-cm-high). Experimental variables were the pool temperature, the cadmium-pool mixing speed, and the salt-pool mixing speed. The impellers that mixed the cadmium and salt pools were mounted on a single shaft. Each impeller consisted of four paddles approximately 3-cm-wide. The paddles were approximately 6-mm-high in the cadmium pool and 13-mm-high in the salt pool. At a mixing speed of 300 rpm, the cadmium vaporization rate increased from 8 .mu.g/h.cm.sup.2 at 451.degree. C. pool temperature to 40 .mu.g/h.cm.sup.2 at 500.degree. C. and to 290 .mu.g/h.cm.sup.2 at 608.degree. C. An equally strong dependence of vaporization rate on mixing speed was observed. At 500.degree. C., the cadmium vaporization rates in .mu.g/h.cm.sup.2 were 0.43 at 0 rpm, 7.7 at 50 rpm, 9.3 at 150 rpm, 19 at 200 rpm, 24 at 250 rpm, 40 at 300 rpm, and 67 at 500 rpm.
It is an object of the present invention to provide a process for gas cleanup to remove at least one metallic contaminant present as a vapor in a gas stream.
Another object of the present invention is to provide a gas cleanup process that controls saturation levels so that essentially no particulates are formed.
Yet another object of the present invention is to provide a gas cleanup process that avoids the requirement of a filter and concomitant problems with plugging, replacement, and disposal.
It is an object of the present invention to provide a system to reduce the cadmium concentration in an electrorefiner cover gas.